package com.business.common.utils;

import java.sql.Timestamp;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;

/**
 * 解决高并发下 System.currentTimeMillis() 的性能问题
 * 具体差别可以运行下面的main方法
 * @author wtm
 *
 */
public class SystemClock {

	private final long period;

	private final AtomicLong now;

	private SystemClock(long period) {
		this.period = period;
		this.now = new AtomicLong(System.currentTimeMillis());
		scheduleClockUpdating();
	}

	private static SystemClock instance() {
		return InstanceHolder.INSTANCE;
	}

	public static long now() {
		return instance().currentTimeMillis();
	}

	public static String nowDate() {
		return new Timestamp(instance().currentTimeMillis()).toString();
	}

	private void scheduleClockUpdating() {

		ScheduledThreadPoolExecutor scheduler = new ScheduledThreadPoolExecutor(
				1, new ThreadFactory() {

					@Override
					public Thread newThread(Runnable r) {
						Thread thread = new Thread(r, "System Clock");
						thread.setDaemon(true);
						return thread;
					}
				});

		scheduler.scheduleAtFixedRate(new Runnable() {

			@Override
			public void run() {
				now.set(System.currentTimeMillis());
			}
		}, period, period, TimeUnit.MILLISECONDS);
	}

	private long currentTimeMillis() {
		return now.get();
	}

	private static class InstanceHolder {
		public static final SystemClock INSTANCE = new SystemClock(1);
	}
	
	public static void main(String[] args) {
		long start = System.currentTimeMillis();
		for (long i = 0; i < Integer.MAX_VALUE; i++) {
			SystemClock.now();
		}
		long end = System.currentTimeMillis();
		System.out.println("SystemClock Time:" + (end - start) + "毫秒");
		long start2 = System.currentTimeMillis();
		for (long i = 0; i < Integer.MAX_VALUE; i++) {
			System.currentTimeMillis();
		}
		long end2 = System.currentTimeMillis();
		System.out.println("currentTimeMillis Time:" + (end2 - start2) + "毫秒");
	}
}
